Project Summary The overall goal of this project is to understand how microtubule motors kinesin and dynein organize cytoskeleton in interphase cells and, as a result, drive cell polarization. Our group recently discovered that major motor proteins, kinesin-1 and cytoplasmic dyneins, in addition to moving many types of cargoes along microtubules, have a new function: they define the distribution and dynamic behavior of two classes of cytoskeletal filaments, microtubules and intermediate filaments in interphase cells. We have shown that both microtubules and intermediate filaments are being transported as polymers by motor proteins. This transport defines general organization of the cytoskeleton, and, as a result, cell polarity. Three biological models of polarization studied in this proposal are: (i) Drosophila neurons, where movement of microtubules by kinesin-1 drives initial extension of axons and dendrites, and cortical dynein sorts axonal microtubules into uniformly polarized arrays; (ii) Drosophila oocytes, where transport of microtubules by kinesin-1 drives rotation of cytoplasm for localization of polarity determinants critical for the developing embryo, and cytoplasmic dynein transports microtubules in nurse cells and from nurse cells to the oocyte and (iii) transport of intermediate filaments along microtubules in mammalian cells, which is important for directed cell migration.